Interpreting geophysical well logs

Interpreting Geophysical Well Logs

Hassan Harraz

[email protected]

mailto:[email protected]

Lecture itemsHistorical Aspect

Types of Logsa) Gamma Ray

b) Sonic

c) Density/Neutron

d) Caliper

e) SP (spontaneous potential)

f) Resistivity (Induction)

Self Potential Log* Theory of measurement.

-Shale-base line& Sand line-SSP, PSP and SP log readings

* Factors affecting on log readings.* Applications.

-Resistivity Logs* Definition.

* Types

* Units& Presentation.* Theories of measurement.* Factors affecting on log readings.* Applications.

2

Historical Aspect Schlumberger brothers, Conrad and Marcel, are credited

with inventing electrical well-logs.

On September 5, 1927, the first “well-log” was created in a

small village named Pechelbroon in France.

In 1931, the first SP (spontaneous potential) log was

recorded. Discovered when the galvanometer began

“wiggling” even though no current was being applied.

The SP effect was produced naturally by the borehole mud

at the boundaries of permeable beds. By simultaneously

recording SP and resistivity, loggers could distinguish

between permeable oil-bearing beds and impermeable

nonproducing beds.

3

Types of Logs

a) Gamma Ray

b) Sonic

c) Density/Neutron

d) Caliper

e) SP (spontaneous potential)

f) Resistivity (Induction)

4

Well LoggingIs a technique used for formation evaluation

to determine the size of the reservoir and the

amount of oil and gas in place.

The following parameters can be estimated

from different types of logging tools:

1) Borehole Hole Diameter.

2) Reservoir Thickness.

3) Porosity.

4) Water Saturation.

5) Rock Type (Lithology).

5

Logging tools classification Based on the function, logging tools can be classified as follow:

1) Formation Fluid Indicators:

▪Induction

▪Laterolog

▪Microfocused and microresistivity devices

2) Formation property-lithology Indicators:

▪Acoustic

▪Density and lithologic density

▪Neutron

▪Gamma ray

3) Layer geometry Indicators:

▪Dipmeter

▪Borehole gravimeter

4) Auxiliary tools:

▪Spontaneous potential

▪Caliper

5) Specialty Tools:

▪Nuclear Magnetic Resonance

▪Dipole

▪Geochemical Tools

6

a) Gamma Ray LogThe gamma ray measures the natural radioactivity of the rocks, and does not measure any

hydrocarbon or water present within the rocks.Shales: radioactive potassium is a common component, and because of their cation exchange

capacity, uranium and thorium are often absorbed as well. Therefore, very often shales will display high gamma ray responses, while sandstones and

limestone will typically show lower responses.

Shale is usually more radioactive than sand or carbonate, gamma ray log can be used tocalculate volume of shale in porous reservoirs. The volume of shale expressed as a decimalfraction or percentage is called Vshales.

Calculation of the Gamma Ray Index (IGR ) is the first step needed to determine the volume ofshale from gamma ray log.

The gamma ray log has several nonlinear empirical responses as well a linear responses. Thenon linear responses are based on geographic area or formation age. All non linearrelationships are more optimistic that is they produce a shale volume value lower than that fromthe linear equation. Linear response :

Where:

IGR =Gamma ray index

GRLog = gamma ray record from log

GRmin = gama ray for clean sand

GRmax = gamma ray for shale

7

a) Gamma Ray

8

The scale for GR is in

API (American

Petroleum Institute) and

runs from 0-125

units There are

often 10 divisions in a

GR log, so each

division represents 12.5

units.

Typical distinction

between a

sandstone/limestone

and shale occurs

between 50-60 units.

Often, very clean

sandstones or

carbonates will display

values within the 20

units range.

a) Gamma Ray➢For very hard compacted formation at depth of 8,000 ft

or more, gamma ray index is considered equal to shalevolume:

Vsh= IGR

➢For tertiary sediment rocks at depth of less than 4,000ft, the shale volume is:

Vsh= 0.083(23.7IGR

-1)

➢For older rocks at depth of 4,000-8,000 ft, the shalevolume is:

Vsh= 0.33(22IGR

-1)

9

d) Caliper Log Caliper Logs record the diameter of the hole. It is very useful in relaying information about the quality of the

hole and hence reliability of the other logs. An example includes a large hole where dissolution, caving or falling of the rock wall occurred, leading to

errors in other log responses. Most caliper logs are run with GR logs and typically will remain constant throughout.

Borehole geometry is controlled by:

➢ Lithology

➢Mud type

➢ Formation Properties

➢ In-situ stresses

Borehole size can be determined from caliper log. Caliper log can be an indication to one of the following cases:

1) Gauged hole: diameter of hole is about equal to the bit size Hard well consolidated and impermeable formation. borehole diameter = drill bit size

2) Increased borehole diameter which means:a) Washout: general drilling wear, especially in shaly zones and dipping beds, both caliper larger than bit

size, considerable vertical extent .

b) Keyseat: asymmetric oval holes, formed by wear against the drill string at points where the borehole inclination changes (doglegs) .

c) Breakout: similar to keyseat but not due to doglegs, small brittle fractures due to existing stress regime of the country rock.

Unconsolidated formation borehole diameter > drill bit size

3)Decreased borehole diameter means:

a) Generally due to formation of mud cake

Mud cake thickness = (bit size diameter – caliper diameter reading)/2

b) mud cake formation indicates permeability and involves loss of mud filtrate into a permeable formation – invasion

Permeable formation borehole diameter < drill bit size

10

b) Sonic (or Acoustic) LogSonic logs (or acoustic) measure the porosity of the rock.

Hence, they measure the travel time of an elastic wave through a formation (measured in ∆T- microseconds per meter).

Intervals containing greater pore space will result in greater travel time and vice versa for non-porous sections.

Wyllie’s time average equation can be used to determine porosity:

Where

t = log reading (s/ ft)

tma = transittime for the rock type (matrix)

tf =transittime for the fluid filling pores (usually189s/ft)

11

Sonic logs are used to determine:

1) Determine porosity of reservoir

rock

2) Improve correlation and

interpretation of seismic records

3) Identify zones with abnormally

high pressures

4) Assist in identifying lithology

5) Estimate secondary pore space

6) Indicate mechanical integrity of

reservoir rocks and formations

that surround them (in

conjunction with density data)

7) Estimate rock permeability

Must be used in combination with

other logs, particularly gamma

rays and resistivity, thereby

allowing one to better understand

the reservoir petrophysics.

12

HW Q.8:

From the gamma ray log, the record is 200 API, gamma ray

for shale zone is 120 API and gamma ray for clean sand is 40

API. Calculate the gamma ray index and shale volume if the

rocks at depth 3,500 ft and 7,500 ft.

Q.9: Sonic log reading t=100s/ft, tma = 80s/ft, tf=190s/ft. Calculate porosity.

13

QuickTime™ and a decompressor

are needed to see this picture.

c) Density Log The density log belongs to the group of active nuclear tools, which contains a

radioactive source and two detectors. The Gamma Ray tool, which is a passive

nuclear tool, contains no source and can only measure the natural radiation in

the formation. The radioactive source is applied to the wellbore wall in a

shielded sidewall skid and emits medium gamma rays into the formation. The

gamma ray waves may be thought of as energy particles. As these energy

particles (photons) collide with the electrons in the formation, the gamma ray

loses some of its energy to the electron. This is called Compton scattering. The

denser the formation, the more electrons are presented, and more energy is

lost due to collisions. If the matrix density is known, then the energy loss is

directly related to porosity.

Density logs measure the bulk electron density of the formation, and is measured in kilograms per cubic meter (gm/cm3 or kg/m3).

Thus, the density tool emits gamma radiation which is scattered back to a detector in amounts proportional to the electron density of the formation. The higher the gamma ray reflected, the greater the porosity of the rock.

Electron density is directly related to the density of the formation (except in evaporates) and amount of density of interstitial fluids.

Helpful in distinguishing lithologies, especially between dolomite (2.85 kg/m3) and limestone (2.71 kg/m3).

17

c) Density Log

18

c) Density Log

19

d) Neutron LogsdNucleus of all elements except hydrogen have neutrons. Neutrons have same mass as

protons but no charge. Their small size and electrical neutrality make neutrons ideal

projectiles for penetrating matter. Two categories of neutron sources are found in the

logging industry: chemical and pulsed sources. Chemical sources are composed of two

elements in intimate contact that continuously emit neutrons, usually Plutonium/Beryllium

or Americium/Beryllium. Such sources need to be heavily shielded when not in use.

Pulsed sources incorporate a neutron accelerator and a target, and can be activated by

simply switching on the accelerator. This source is used for pulsed neutron logging and in

tools that measure inelastic neutron collisions .

Neutron Logs measure the amounts of hydrogen present in the water atoms of a rock, and

can be used to measure porosity. This is done by bombarding the the formation with

neutrons, and determing how many become “captured” by the hydrogen nuclei.

Because shales have high amounts of water, the neutron log will read quite high

porosities- thus it must be used in conjunction with GR logs.

However, porosities recorded in shale-free sections are a reasonable estimate of the pore

spaces that could produce water.

It is very common to see both neutron and density logs recorded on the same section, and are often shown as an overlay on a common scale (calibrated for either sandstones or limestone’s).

This overlay allows for better opportunity of distinguishing lithologies and making better estimates of the true porosity.

* When natural gas is present, there becomes a big spread (or crossing) of the two logs, known as the “Gas Effect”. 20

d) Neutron Logs

21

The following equation can be used to determine porosity from density log:

HW Q.10 :

The bulk density reading from density log is (2.2 gm/cc). The density of matrix is (2.45 gm/cc) and fluid density is (1.035 gm/cc). The density reading from neutron log is (15%). Calculate formation density.

22



Example of dolomite overlying limestone, as

distinguished by the neutron/density.

e) Resistivity Log Resistance is the opposition offered by a substance to the

passage of electric current. Resistivity is the resistancemeasured between opposite faces of a unit cube of thesubstance at specified temperature. Resistivity is measured inohm-meter2/meter, more commonly shortened to just ohm-meter.

Resistivity logs do not always measure resistivity directly.

➢ Some resistivity logs (actually induction logs) measuresconductivity instead which is the reciprocal of resistivity.

Induction logs are used in wells drilled with a relatively fresh-water mud (low salinity) to obtain more accurate value of trueresistivity.

30

e) Resistivity (Induction) Resistivity logs record the resistance of interstitial fluids to the flow of

an electric current, either transmitted directly to the rock through an electrode, or magnetically induced deeper into the formation from the hole.

Therefore, the measure the ability of rocks to conduct electrical currents and are scaled in units of ohm-meters.

On most modern logs, there will be three curves, each measuring the resistance of section to the flow of electricity.

Porous formations filled with salt water (which is very common) have very low resistivities (often only ranging from 1-10 ohms-meter).

Formations that contain oil/gas generally have much higher resisitivities (often ranging from 10-500 ohms-meter).

With regards to the three lines, the one we are most interested in is the one marked “deep”. This is because this curve looks into the formation at a depth of six meters (or greater), thereby representing the portion of the formation most unlikely undisturbed by the drilling process.

One must be careful of “extremely” high values, as they will often represent zones of either anhydrite or other non-porous intervals.

31

The resistivity of a rock (R) is given by:R = r (A / L)

Where:r = resistance (ohms) = E / I

A = Cross sectional area (meters2) L = Length (meters)R = resistivity (ohm-meters)E = Voltage (Volt)I = current (Amp)

Factors that influence Resistivity of Natural Porous Media:

1) Salinity of water2) Porosity3) Stress

4) Temperature

5) Pore geometry6) Rock Composition7) Wettability

32

Formation Water Resistivity (Rw):

Formation water resistivity can vary widely from well to well. It can be estimated by thefollowing methods;

➢Chemical analysis of produced water

➢Direct measurement in resistivity cell

➢Using Empirical equations

The best method is direct measurement of resistivity.

Chemical analysis:

Resistivity of water is controlled by amount and type of ions present and temperature.Salinity is a measure of concentration of dissolved salts in water and is generally expressed asparts per million, grains/gallon or grams/liter.

1 grain/gallon = 17.118 ppm = 0.017118 grams/liter

NaCl is the most common dissolved salt in formation water; the concentration of otherdissolved ions is generally converted to equivalent concentration of sodium chloride;

Where;

C = equivalent concentration of NaCl.

Mi=weight multiplier (can be estimated from graph)

Ci= concentration of each ion.

33

Based on equivalent

concentration of NaCl

and temperature,

formation water resistivity

can be determined using

graph.

The following equation

also can be used to

calculate (Rw).

35

Formation Resistivity (Ro)

The resistivity of the formation saturated 100% with

formation water.

Archie equation:

Where:

FR = Formation factor

Formation porosity or the void space in the formation can

be determined from formation factor using the following

equation:

36

HWQ.1

The chemical analysis of formation water as follow;

Room temperature=75oF

Calculate formation water resistivity at 75, 125 and 150oF.

Q.2

Formation water contains 10,000 ppm of NaCL, 15,000 ppm of MgSO4 and 8,000 ppm of CaCl2. Calculate the resistivity at formation temperature 200oF.

Q.3

Calculate formation water resistivity at 150oF if the concentration of NaCl 50,000, 100,00 1nd 150,000 using graph and equations.

Q.4

If the formation resistivity in the above cases (Q.3) is 2.4 Ω-m at 225oF and the cementation factor is 2. Calculate the porosity for each case.

37

Ion Concentration (ppm)

Na 14,000Cl 12,000Mg 10,000Ca 8,000SO4 11,000

True Resistivity (Rt):

The resistivity of the formation at any saturation of water less

than 100% when the hydrocarbon displaces some water

from pore space in the formation. The relationship between

formation resistivity (Ro) and true formation resistivity (Rt)

can be represented by resistivity index:

Where: IR = resistivity index

Water saturation (Sw) which is defined as the percentage of

the pore volume filled with water can be determined from the

following equation :

Where: n = saturation exponent ≈2

38

HW Q.5

Calculate porosity and water saturation if the formation factor is 15, true formation resistivity 10 Ω-m and the concentration of the formation water at 75oF is 60,000 ppm. Use m=n=2 and formation temperature 200oF.

Q.6

The resistance cylindrical core having 3 in diameter and 10 in height saturated 100 % with formation water is 10 Ω. The resistance of the core is increased to 85 Ω when oil is injected to it. Calculate water saturation of the core after the injection of oil.

39

f) SP (Spontaneous Potential)• The SP log records the electric potential between an electrode

pulled up a hole and a reference electrode at the surface.• This potenital exists because of the electrochemical differences

between the waters within the formation and the drilling mud. • The potenital is measured in millivolts on a relative scale only

since the absolute value depends on the properties of the drilling mud.

• In shaly sections, the maximum SP response to the right can be used to define a “Shale Line”.

• Deflections of the SP log from this line indicates zones of permeable lithologies with interstitial fluids containing salinities differing from the drilling fluid.

• SP logs are good indicators of lithology where sandstones are permeable and water saturated.

• However, if the lithologies are filled with fresh water, the SP can become suppressed or even reversed. Also, they are poor in areas where the permeabilities are very low, sandstones are tighly cemented or the interval is completely bitumen saturated (i.e., oil sands).

40

f) SP (Spontaneous Potential) The spontaneous potential (SP) log is a measurement of the natural potential

difference or self-potential between an electrode in the borehole and a referenceelectrode at the surface . It represents a recording of naturally occurring physicalphenomenon in in-situ rocks.

The SP curve records the electrical potential (voltage) produced by the interaction offormation water, drilling mud and shale. Though relatively simple in concept, the SPcurve is quite useful for a number of things:1) Differentiates potentially porous and permeable reservoir rocks2) from nonpermeable shales3) Defines bed boundaries and correlation of beds4) Aids in lithology identification5) Detection of hydrocarbon from suppression of SP response6) Permits determination of formation water resistivity, Rw7) Gives semi-quantitative indication of bed shaliness

Three factors are necessary to produce an SP current:

1) a conductive fluid in the borehole,

2) a porous and permeable bed surrounded by an impermeable formation, and

3) a difference in salinity (or pressure) between the borehole fluid and the formation fluid.

41

Resistivity of drilling mud filtrate (Rmf): ➢The resistivity of drilling mud filtrate

which is normally observed in thepermeable layers.

➢The SP deflection is a reflection ofcontrast between the mud filtrateand connate water resistivity.

➢The deflection is said to be normalor -ve when the mud filtrate is moreresistive than the connate waterand is reverse or +ve when the mudfiltrate is less resistive that theconnate water. It is quite commonto find fresh water in shallow sandsand increasingly saline water asdepth increases. Such a progressionis shown in the figure, where SPappears deflecting to left deep inthe well but is reversed near to thesurface.

42

43

SP (spontaneous potential)

44

1- Electrokinetic Potential (can be neglected)

48

2- Electrochemical Potential 1) Membrane Potential

2) Liquid Junction Potential

Shale Baseline and SSP: SP has no absolute values and thus treated quantitatively and qualitatively in terms of

deflection, which is the amount the curve moves to the left or to the right of a defined zero. Thedefinition of the SP zero, called shale baseline, is made on thick shale intervals where the SPcurve does not move. All values are related to the shale baseline.

The theoretical maximum deflection of the SP opposite permeable beds is called the static SPor SSP. It represents the SP value that would be measured in an ideal case with the permeablebed isolated electrically. It is the maximum possible SP opposite a permeable, water-bearingformation with no shale.

The SSP is used to calculate formation-water resistivity (Rw).

SP = -K log(Rmfe/Rwe)

SP= SP value: this should be the SSP

(Rmf)e = equivalent mud filtrate resistivity: closely related to Rmf

(Rw)e = equivalent formation water resistivity: closely related to Rw

K = temperature-dependent coefficient = 61+ 0.133 * T

T= formation temperature (°F)

SP value measured is influenced by:

• Bed thickness

• Bed resistivity (Rmf, Rw, )

• Borehole and invasion

• Shale content

• Ratio of Rmf/Rw (amplitude and sign)

• Temperature

55

Factors affecting SP log measurements

Rmf/Rw (Salinity effect) Fresh mud: negative SP, Saline mud: positive SP.

Shale or clay content Shale reduces SP.

Permeability

Presence of hydrocarbon

Bed thickness: SP decreases when bed thickness decreases.

Invasion: Reduces SP.

Mud filtrate: The magnitude and direction of SP deflection from the shale baseline depends on relative resistivities of the mud filtrate and the formation water.

Resistive formations

56

57

PSP (Pseudo-static SP):the SP value in the water–bearing shalysand zone read from the SP log. SSP (Static SP): the maximum SP value in a clean sand zone.

Clean Laminated Structural Dispersed

63

Q.7

Calculate water formation resistivity

and shale volume if SSP=40 mv and

PSP=15 mv. Reservoir temperature is

250oF and Rmf=0.5 Ω-m.

64

67

Rw

calculation

from SP

Mathematical Calculation of Rw from SSP (modified after Bateman & Konen, 1977)

Rmf at 75oF = Rmf temp

* x (temp + 6.77)/81.77

Correction of Rmf to 75o

K = 60 + (0.133 x Tf)

Rmfe / Rwe = 10 – SSP / K**

Rmfe = (146 x Rmf – 5) / (337 x Rmf + 77)

Rmfe formula if Rmf at 75oF < 0.1

Rmfe = 0.85 x Rmf

Rmfe formula if Rmf at 75o > 0.1

Rwe = Rmfe / (Rmfe / Rwe)

Rw at 75oF = (77 x Rwe + 5) / (146 – 377 x Rwe)

Rw at 75o formula if Rwe < 0.12

Rw at 75oF = - [0.58 – 10

(0.69 x Rwe –0.24)]

Rw at 75oF formula if Rwe > 0.12

Rw at formation temperature = Rw at 75o x 81.77 / (Tf + 6.77)

*Rmftemp = Rmf at a temperature other than 75oF

**The e subscript (i.e. Rmfe) stands for equivalent resistivity.

69

Applications

Differentiation between shaly, clean and shale zones.

Differentiation between Permeable and non-permeable zones.

Calculation of Rw.

Determination of the volume of shale.

For correlation purposes

For sedimentological analysis and facies studies.

70

Notice how the shale baseline shows a

distinctive drift with depth. This

characteristics is commonly caused by an

increases in relative oxidation of the rocks

that are close to the land surface. The

highest sandstone in the well has a muted

deflection on the SP log as compared with

the lower sandstones. This contrast is an

immediate indication that water in the upper

sandstone may be significantly fresher than

waters of the lower sandstone. In other

wells it is not uncommon to see sandstone

units where the SP deflection goes to the

right of the shale baseline. In these

instances, the drilling mud filtrate is salter

than the formation water.

73

74

A good example of thisphenomenon is shown inthe figure attached. In theupper sandstone, "U", theSP log shows a deflectionto the right, indicatingformation water to befresher than the drillingmud, while in the lowersandstone, "L", thedeflection is to the left,showing the formationwater to be more saline.

Flow chart from oil-industry log

analysis to estimate formation

water resistivity, Rw, in deep

formations from the SP log. RMF

is mud filtrate resistivity measured

at temperature Tmf and recorded

on the log header; Tf is the

temperature of the formation,

generally estimated by

interpolating between the bottom-

hole temperature (BHT) at total

depth (TD) and mean annual

temperature at the surface; SSP is

the static self-potential measured

on the log between the "clean

line" and "shale line" in millivolts

(mv) and with associated sign

(positive or negative).

75

Qu ic k Tim e™ and a d ec o m pres s or

are ne ede d to s e e th is p ic tu re.

Interpreting geophysical well logs

Science

Transcript of Interpreting geophysical well logs